The present invention relates generally to welding and more specifically to a control system for use in arc welding operating in the short circuit (or dip) transfer mode.
In a typical arc welding system operating in the dip transfer mode, a welding circuit is established which includes a consumable electrode, a workpiece and a power source, The electrode is generally a solid wire and not only conducts the electric current and sustains the arc, but also melts and supplies filler material into the joint. A shielding gas such as carbon dioxide or blends of argon with carbon dioxide and/or oxygen may be supplied during the welding process to support the arc and prevent the molten metal reacting with oxygen and nitrogen in ambient air.
During the arcing phase of the welding cycle, a molten metal droplet forms on the end of the electrode. As the electrode is advanced from the contact tip, the metal droplet engages the molten metal pool formed in the workpiece creating a short circuit. The arc is actually quenched at this point and the current rises at a rate determined by the power source characteristics. The increase in current causes an electromagnetic pinch force to be applied to the molten droplet material that forms a bridge between the electrode and the workpiece. The applied pinch force assists in the promotion of the bridge rupturing, so that molten material is transferred to the workpiece and the arc is re-established. However, the high current at the time of rupture often causes the bridge to rupture with an explosive force, thereby resulting in welding spatter. Spatter is undesirable in the welding process as it diminishes the weld quality and results in additional cleaning of the weld site, thereby increasing both the cost and time of production of the weld.
Sophisticated power sources have been developed with spatter control systems which minimise the spatter by ensuring the current is turned off immediately before an impending bridge rupture is detected. However, these control systems are not widely applicable as the majority of power sources are not capable of switching the current off fast enough prior to rupturing occurring.
An aim of the present invention is to provide a control method and system for use in welding which can reduce spatter and which is able to be used with conventional power supplies, A further aim of the invention is to provide a welding power supply incorporating this improved control method and system.
In a first aspect, the invention provides a method of controlling an arc welding system operating in the dip transfer mode, the welding system including a power source and a consumable electrode which in operation is operative to be advanced into contact with a workpiece, the welding system being operative to create a welding circuit which is energised by said power source and which has a welding cycle comprising an arcing phase where the electrode is spaced from said workpiece and an arc is generated across said space, the arc being operative to form a molten droplet on the end of the electrode, and a short circuit phase where the electrode is in contact with said workpiece, the welding cycle changing from the arcing phase to the short circuit phase on contact of the molten droplet with the workpiece, and changing from the short circuit phase to the arcing phase after rupturing of a bridge of molten material formed between said electrode and said workpiece, the method including the steps of:
(i) conditioning the welding system to form a molten droplet on the electrode end during the arcing phase which is above a predetermined threshold size so that bridge rupturing can occur during the short circuit phase without requiring high current during the short circuit, and
(ii) controlling the current output from the power source during the short circuit phase.
In a second aspect, the invention provides a system for controlling an arc welding system operating in the dip transfer mode, the welding system including a power source and a consumable electrode which in use is operative to be advanced into contact with a workpiece, the welding system being operative to create a welding circuit which is energised by said power source and which has a welding cycle comprising an arcing phase where the electrode is spaced from said workpiece and an arc is generated across said space, the arc being operative to form a molten droplet on the end of the electrode, and a short circuit phase where the electrode is in contact with said workpiece, the welding cycle changing from the arcing phase to the short circuit phase on contact of the molten droplet with the workpiece, and changing from the short circuit phase to the arcing phase after rupturing of a bridge of molten material formed between said electrode and said workpiece, the control systems including:
(i) conditioning means operative to condition the welding system to form a molten droplet on the electrode end during the arcing phase which is within a predetermined threshold size range so that bridge rupturing can occur during the short circuit phase without requiring high current during the short circuit, and
(ii) current control means operative to control the current output from the power source during the short circuit phase,
In accordance with the present invention, the control system uses the size of the droplet to obviate the need for a high current to initiate bridge rupturing. In the present system, because of the size of the droplet, rupturing is able to result primarily from the surface tension at the droplet-pool interface. Therefore, with the control system, the current is able to be maintained at a relatively low level throughout the short circuit phase. In this regard, it has been found that utilising the systems and methods of the invention, bridge rupturing can occur using current levels of 30-40% of the natural short circuiting current. As the natural short circuiting current is typically in the vicinity of 350 amps, using the present invention bridge rupturing may occur in the vicinity of 150 amps.
A control system according to the present invention is therefore able to significantly reduce spatter. Further, the system has the substantial benefit over existing spatter control systems as it does not require fast xe2x80x9cturn-offxe2x80x9d of the current prior to bridge rupture to reduce spatter and is therefore capable of being used on a wide range of power sources.
The molten droplet formed during the arcing phase should not be excessively big as this can lead to process instability. In particular, if the droplet is too big during the arcing phase, it may shift from a central position on the electrode tip which will adversely affect its positioning in the contact area of the workpiece and may also lead to localised arcs forming particularly when CO2 shielding gas is used. Further, if the droplet is too big, it may separate prematurely from the electrode.
In a preferred form, the control system is operative to condition the welding system to produce a molten droplet within the threshold size range by applying a current pulse during the arcing phase. This current pulse is specifically designed to increase the size of the molten droplet but is less than required to cause detachment of the droplet from the electrode. Preferably following the current pulse, the current is maintained during the arcing phase at a level which will produce the required heat input to ensure that the droplet remains molten and workpiece plate fusion occurs.
Preferably during the short circuit phase, the current is maintained or clamped below a predetermined level. If required, the current may be reduced after the short circuit is detected to assist the wetting in of the molten droplet. Once the wetting in period is completed, the current may then be increased to its clamp level where it is held until the bridge ruptures. After detecting the rupture, the welding process enters the subsequent arcing phase where the current pulse is again applied to grow the droplet to its threshold size.
The size of the molten droplet formed during the arcing phase will determine the level of current, if any, that is required to cause bridge rupturing during the short circuit phase. Accordingly, the value of the threshold size of the droplet will determine the amount of current that is required. It is to be appreciated that both the threshold size of the droplet and the current level during the short circuit phase may vary within a considerable range whilst still achieving the prime objective of the invention to minimise spatter.
In one form, the control system is able to control the droplet size prior to short circuit to ensure that it reaches an optimum level, The size of-the molten droplet is determined or estimated in real time during the welding cycle, In this way, the control system is able to determine when the droplet has reached its threshold level. The advantage of this arrangement is that the parameters established by the control system to condition the welding process to form the droplet may be appropriately set to optimise droplet growth in the arcing phase to produce a quality weld.
Alternative methods exist for the real time estimation of droplet size.
One method of droplet size estimation requires firstly that that the contact tip to workpiece distance (CTWD) is estimated. This is done by measuring the minimum short circuit resistance during the short circuit, the duration of the short circuit, and applying two correction factors to estimate CTWD:                     CTWD        =                  a          ⁢                      R                          sc              ⁢                              xe2x80x83                            ⁢              _              ⁢                              xe2x80x83                            ⁢              min                                xc3x97                      b                          T              sc                                                          Units:meters            
where b is the time-dependent correction factor, and a is the resistance-dependent correction factor. The transit time, which is the time taken for an element of material in the electrode to travel from contact tip to workpiece, can then be calculated as:                               T          transit                =                  CTWD                      Wire            ⁢                          xe2x80x83                        ⁢            Feed            ⁢                          xe2x80x83                        ⁢            Speed                                              Units:seconds            
The amount of electrode preheating, which affects the rate at which material at the electrode tip melts, can then be estimated using the action integral:                     Action        =                              ∫                          t              -                              T                transit                                      t                    ⁢                                                    i                2                            ⁡                              (                t                )                                      ⁢                          ⅆ              t                                                          Units:AAs            
where t is the present time. The instantaneous melting rate of the electrode is then,                               MR          ⁡                      (            t            )                          =                  α                      1            -                          β              xc3x97              Action                                                          Units:meters/sec            
where xcex1 and xcex2 are the arcing and resistive melting rate constants of the electrode being used, Integrating the melting rate over the duration of the arcing period calculates the length of electrode L(t) which has been molten during the arcing:                               Δ          ⁢                      xe2x80x83                    ⁢                      L            ⁡                          (              t              )                                      =                              ∫                          t              =                              start                ⁢                                  xe2x80x83                                ⁢                of                ⁢                                  xe2x80x83                                ⁢                arc                                      t                    ⁢                                    MR              ⁡                              (                t                )                                      ⁢                          ⅆ              t                                                          Units:meters            
The final step is to estimate the droplet diameter by converting the cylindrical element of electrode of length L into a sphere of diameter ds.
The above method of estimating droplet size as it is being formed allows the control system to terminate conditioning of the welding system, such as terminating the arcing current, when a target size has been reached.
An alternative method of droplet size estimation is to simply measure the elapsed time of each weld cycle and estimate the droplet size assuming the wire feed rate remains exactly constant:
xcex94Lavg=Wire Feed Speedxc3x97Tcyclexe2x80x83xe2x80x83Units: meters 
An average of several values may need to be taken to obtain a reliable estimate. Unlike the more complex method, the size can be estimated only after the short circuit has started (ie at the end of droplet formation), No pre-emptive action can be taken by the controller to limit droplet size.
Once the droplet size is estimated by either method, the controller can compensate for insufficient droplet size by changing the conditioning parameters such as increasing the steady-state arcing current in the subsequent cycle, in order to ensure that a suitable size is achieved before the next short circuit. Alternatively, if the droplet size is excessive, the steady state arcing current can be reduced so that the short circuit frequency is increased. This makes the process more appealing to a manual operator.
In another form, the control system does not measure the droplet size, but rather an estimate is initially made as to what conditioning of the system is required to suit the welding process parameters to produce a droplet of appropriate size.
In this arrangement, preferably the control system provides feedback information as to whether an appropriate weld has been made in any cycle. This information enables the conditioning of the system in the arc phase to be varied or the short circuit current level to be altered in subsequent cycles if an appropriate weld is not formed in any cycle. In this way, the system is adaptive and able to optimise the control system parameters.
In one form, the feedback information may be the duration of the short circuit phase, If the short circuit falls outside a specified range (that is, if it is too short, or too long), the conditioning parameters during the arcing phase, or the short circuit current, may be appropriately altered.
Preferably the control system also has the facility to increase the current in a weld cycle if bridge rupturing has not occurred in a specified time. This facility to be able to increase the current acts as a fallback to ensure that that detachment occurs within the specified parameters.
The parameters of the control system, including the pulse parameters, the threshold size of the droplet, the current clamp value during the short circuit, will vary depending on the parameters of the welding system, such as the composition of the shielding gas, the thickness of the wire, and characteristics of the power source. Optimum values for these parameters in any situation can be established through experience and/or through use of adaptive techniques where the weld quality is calculated over successive cycles of the welding process. As an example, however, the range of current pulse amplitudes will typically be in the order of 200 to 400 amps with the pulse width duration being in the range of 0.5 to 3.0 msec. The short circuit clamping range may vary significantly depending on the arcing current and the work to tip distance. The clamping level may vary from 25 to 200 amps, but preferably is in the range of 50 to 150 amps.
Preferably the droplet diameter range is in the order of 1.2 to 2.5 mm in diameter assuming a spherical droplet. Within this range, the current clamp levels is preferably between 50 to 150 amps for argon based gasses and 75 to 200 amps for CO2 gas shielding.
Preferably the short circuit time is in the range of 2.0 to 4.8 msec. If the duration exceeds between 5 to 7 msec, the current clamp is preferably released for the individual short circuit and then returned to the pre-set value immediately after the short circuit has cleared. Preferably this occurs at 5.0 msec for an argon+20% CO2 gas shield and at 6.0 msec for a CO2 gas shield.
Preferably the arcing time is between 10 msec and 20 msec. If the upper value is exceeded, particularly in the case of CO2 shielding, the current may be reduced to force the next short circuit to occur.
In yet a further aspect, the present invention relates to a power supply which incorporates a control system according to any form as described above. The power supply is characterised in that it incorporates a control system which is operative to control the output of the power source such that, in a welding system in which the power source is connected, the power source is operative to condition the welding system to form a molten droplet during an arcing phase of the welding cycle which is within a predetermined threshold size range.